Solid hydrogen fuel elements and methods of making the same

ABSTRACT

A hydrogen fuel element ( 10, 110, 210 ) includes a heat-generating pyrotechnic charge ( 12 ) which comprises any suitable pyrotechnic material and an ammonia borane encasement ( 16, 116 ). The encasement partly (encasement  16 ) or wholly (encasement  116 ) encases the pyrotechnic charge ( 12 ). An ignition train ( 14 ) is powered by electrical leads ( 28   a   , 28   b ) to ignite pyrotechnic charge ( 12 ) to heat both the ammonia borane binder it contains and the encasement ( 16, 116 ), which itself includes or is made entirely of ammonia borane. Hydrogen is evolved from the heated ammonia borane binder and encasement. The hydrogen fuel element ( 10, 110 ) may be encased within a suitable housing ( 30 ) which may be made of a carbon open-cell foam.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority of provisional PatentApplication Ser. No. 60/777,212, entitled “Solid Fuel Elements, HydrogenCartridges Including the Same, and Methods of Making the Same”, filed onFeb. 27, 2006.

GOVERNMENT CONTRACTS

Work which resulted in this invention was done in connection withContract No. ANG004588 with General Dynamics Armament and TechnicalProducts (“GDATP”) under Government Contract No. W909MY-05-C-0017between the United States Government and GDATP.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention concerns ammonia borane solid fuel elements whichprovide a source of hydrogen gas. In particular, the present inventionprovides a solid fuel element comprising a heat charge and an ammoniaboron encasement for the pyrotechnic charge.

2. Related Art

The use of binders in energetic materials is well known in the art,including the use of polybutadienes, fluoroelastomers, polyesters andcopolymers of the foregoing. Some binders are reactive and some arenon-reactive, the latter having the effect of diluting the energeticmaterial so that a lesser energetic output per weight of bonded(binder-containing) energetic material is attained. Thermosetting bindersystems are mixed as liquid monomers with chemical crosslinking agentspoured into molds containing the energetic material and then heated andcured. Powder-based binders are dissolved in an appropriate solvent andthe binder precipitates out of solution such that it coats the particlesof energetic material. Polytetrafluoroethylene (“PTFE”) provides asolventless system in which the PTFE liquefies and flows under pressureto coat and bond the particles of energetic material.

In some cases it is desired to embed particles of energetic materialwithin the binder to form a hydrogen solid fuel element as a coherentbody comprising a binder matrix having particles of energetic materialdisposed therein. In other cases it is desired to encapsulate thereactive material within an encasement comprised of the binder. Theamount of binder used determines the desired mechanical properties ofthe resulting product. In any case, whether a reactive or non-reactivebinder is utilized, the binder is not a source of hydrogen andconsequently reduces the fuel element's gravimetric efficiency (weightof hydrogen produced per unit weight of the fuel element). In the caseof hydrogen-generating solid fuel cartridges, a pyrotechnic or otheractivating charge is juxtaposed with a hydrogen source, such as apyrolytic hydride, e.g., ammonia borane, so that ignition of theactivating charge decomposes the pyrolytic hydride to release hydrogengas. Those skilled in the art will appreciate that in many applicationsit is highly desirable to maximize the hydrogen output per unit weightof cartridge, that is, to provide a hydrogen-generating material of highgravimetric efficiency. In such cases, it is necessary to minimize theamount of material in the cartridge which does not generate hydrogen. Asnoted above, conventional prior art binders, whether reactive orunreactive, do not generate hydrogen and of course inert containers usedto house the solid fuel cartridge produce no hydrogen and furtherdecreases gravimetric efficiency.

Published U.S. Patent Application US 2003/0180587 A1 of Peter BrianJones et al. for “Portable Hydrogen Source” discloseshydrogen-generating elements comprising a pellet holder provided withone or more recesses, within which recesses hydrogen-generating pelletsare retained. The pellets may comprise a mixture of ammonia borane andhydrazine bis-borane and optionally other compounds. See page 3,paragraph [0045]. Page 5, paragraph [0063] discloses a combination ofLiAlH₄ and NH₄Cl in one layer and ammonia borane in another layer of thehydrogen-generating pellets.

U.S. Pat. No. 4,315,786 of William D. English et al. for “SolidPropellant Hydrogen Generator” issued on Feb. 16, 1982, discloses aparticulate metal reactant comprised of at least two metals whichundergo an exothermic reaction to form an intermetallic compound inquantity to sustain decomposition of a borane reactant to yield hydrogen(or deuterium). The example at column 3 of the application disclosespressing into a pellet a mixture of fine (micron-sized) nickel andaluminum powders and ammonia borane. A pellet was pressed from thiscomposite mixture and ignited by electrical leads wrapped around thepellet.

U.S. Pat. No. 3,666,672 of Ralph H. Hiltz for “Hydrogen GeneratingCompositions” issued on May 30, 1972, discloses an autogeneouslycombustible composition which liberates hydrogen on burning. Thecomposition contains an alkali metal borohydride and hydrazine sulfatein specified proportions. The mixture is “compressed to form a coherentcompact” (column 1, lines 18-22).

U.S. Pat. No. 4,157,927 of W. M. Chew et al. for “Amine-Boranes asHydrogen Generating Propellants” issued on Jun. 12, 1979, and is typicalof a plethora of prior art disclosing the generation of hydrogen ordeuterium upon combustion of a mixture containing an amine-borane or aderivative thereof and a reactive heat-producing compound or aheat-producing mixture. At column 2, there is disclosed the preparationof such a mixture of fine powders and, after mixing, “the mixed powderis then pressed into pellets using pressure from about 500 to about10,000 pounds total load.” (column 2, lines 25-28).

SUMMARY OF THE INVENTION

The present invention is based, at least in part, on the discovery thatammonia borane can be utilized both as an encasement for the pyrotechniccharge used for heating hydrogen-generating materials to releasehydrogen gas therefrom and as a binder for such pyrotechnic, e.g.,thermite, charges. Ammonia borane itself is a pyrolytic hydride which,as is well known to those skilled in the art, evolves hydrogen whenheated. As a result, the ammonia borane binder or encasement itselfevolves hydrogen when heated by reaction of the pyrotechnic material.

Generally, in accordance with the present invention, ammonia borane isutilized as an encasement for a pyrotechnic activating charge, toprovide a hydrogen-generating fuel element suitable for use in ahydrogen cartridge, which fuel element comprises an encasement whichgenerates hydrogen upon initiation of the pyrotechnic activating charge.

The present invention further contemplates utilizing ammonia borane as abinder for the pyrotechnic material used to heat hydrogen-generatingmaterials used in fuel elements of, e.g., hydrogen generationcartridges. Because the ammonia borane binder itself is a source ofhydrogen, the gravimetric efficiency of the fuel element, and thereforeof the hydrogen-generating device, is increased.

Specifically, in accordance with the present invention there is provideda solid hydrogen fuel element comprising a pyrotechnic charge and acoherent, self-sustaining ammonia borane encasement which partly orfully encases the pyrotechnic charge. The pyrotechnic charge, e.g., athermite, may optionally include a binder such as ammonia borane which,upon being heated to its activation temperature, releases hydrogen.

Another aspect of the present invention provides that the ammonia boraneencasement is made by a process of molding ammonia borane into a hollowshape by placing the ammonia borane into a suitably shaped mold andapplying sufficient pressure to the ammonia borane within the mold torender the encasement as a coherent, self-sustaining body, and placingthe pyrotechnic charge within the encasement.

One aspect of the present invention provides for applying to the ammoniaborane a pressure of at least about 2,000 psi, e.g., a pressure of fromabout 2,000 to about 10,000 psi.

Another aspect of the present invention provides that the pyrotechniccharge is made by a process of admixing an incoherent pyrotechniccharge, e.g., a powder or granular pyrotechnic charge, with a binder,e.g., ammonia borane, to form an admixture of the binder and theincoherent pyrotechnic charge. A related aspect of the inventionprovides that the binder is present in the admixture in a quantity whichis sufficient, upon application to the admixture of suitable pressure,typically at least about 2,000 psi, e.g., from about 2,000 to about10,000 psi, to render the incoherent pyrotechnic charge into a coherent,self-sustaining body. The quantity of binder present, however, must notbe so great as to preclude reliable ignition and burning of thepyrotechnic charge.

Another aspect of the present invention provides a solid hydrogen fuelelement as described above having an ignition train positioned in energytransfer relationship with the pyrotechnic charge.

Yet another aspect of the present invention provides for a hydrogen fuelelement further comprising a housing enclosing the ammonia boraneencasement and the pyrotechnic charge, which housing is pervious tohydrogen gas generated by the fuel element.

A method aspect of the present invention provides for malting a solidhydrogen fuel element comprising subjecting ammonia borane to a pressureof at least about 2,000 psi, e.g., from about 2,000 to about 10,000 psi,to form a coherent, self-sustaining hollow encasement of ammonia borane,and at least partly encasing a pyrotechnic material within theencasement. Optionally, the method may comprise fully encasing thepyrotechnic material within the ammonia borane encasement.

Another method aspect of the present invention comprises admixing abinder with the pyrotechnic material, the binder comprising a pyrolytichydride, e.g., ammonia borane, characterized by evolving hydrogen atleast when heated to a temperature sufficiently high to evolve hydrogenfrom ammonia borane.

The present invention also provides a method of making a solid hydrogenfuel element further comprising mounting an ignition train in signaltransfer communication with the pyrotechnic charge of the fuel element.

Related aspects of the present invention provide one or more of thefollowing: the ignition train may be mounted within the ammonia boraneencasement; the ignition train has an output end which may contact thepyrolytic material; and the encasement and the pyrotechnic charge may beenclosed within a housing which is pervious to the flow of hydrogen gasfrom the interior to externally of the housing.

As used herein and in the claims the term “pyrotechnic material” meansany non-explosive energetic material which may be initiated to evolve ata temperature high enough to release hydrogen from ammonia borane.Preferably, the pyrotechnic material will attain temperatures highenough when initiated to drive off all the hydrogen from a molecule ofammonia borane.

As used herein and in the claims, the term “coherent, self-sustaining”encasement or body means an encasement or body which has been formedinto a given shape and is able to retain that given shape during normalhandling and filling in a manufacturing method as described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of a hydrogen fuel elementsuitable for use in a hydrogen cartridge in accordance with oneembodiment of the present invention;

FIG. 1A is a view, enlarged relative to FIG. 1, of the portion of FIG. 1enclosed by area A;

FIG. 2 is a schematic cross-sectional view in elevation of a hydrogenfuel element suitable for use in a hydrogen cartridge in accordance witha second embodiment of the present invention;

FIG. 3 is a schematic cross-sectional elevation view of a hydrogen fuelelement suitable for use in a hydrogen cartridge in accordance with athird embodiment of the present invention; and

FIGS. 4A-4E schematically illustrate a method of manufacture of a solidhydrogen fuel element in accordance with an embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION AND SPECIFIC EMBODIMENTS THEREOF

As described in the above-mentioned U.S. Published Patent Application2003/0180587 A1, a hydrogen-generating device may comprise a pelletholder provided with a plurality of recesses within whichhydrogen-generating pellets are retained. In other types of devices, thehydrogen-generating fuel may be sized or shaped differently frompellets. Such hydrogen-generating fuel elements (sometimes belowreferred to simply as “hydrogen fuel elements” or “fuel elements”)comprise a heat-generating or other activating charge and a solid sourceof hydrogen, which source may be ammonia borane (NH₃BH₃). A hydrogenfuel element may be equipped with an initiation device to initiate theheat-generating or other activating charge, and a suitable container.

A typical hydrogen fuel element contains an ignition train comprised ofan initiation element, a pick-up charge and an ignition charge. Theignition train is placed in energy-transfer relationship, e.g., incontact with or embedded at least partly within, a main pyrotechniccharge. A solid hydrogen fuel charge is in energy-transfer relationshipwith the main pyrotechnic charge. A housing at least partly encloses thecomponents. Each of the initiation element, pick-up charge, ignitioncharge and main pyrotechnic charge self-propagates a reaction beforeigniting the next material in the train, ultimately the main pyrotechniccharge whose heat output generates hydrogen gas from the solid hydrogenfuel. In general, the energetic materials must remain in closeproximity, if not in intimate contact, in order that each element ignitethe next adjacent element to fully function the device. The housingperforms several functions including, but not limited to, providingstructural support to the components, isolating the energetic materialsfrom their surroundings, and participating in the functional output.Housings can be machined, molded or formed from almost any suitablesolid material.

Referring now to FIG. 1, a hydrogen fuel element 10 comprises aheat-generating pyrotechnic charge 12, an ignition train 14 and anammonia borane encasement 16 which partially encases heat-generatingpyrotechnic charge 12. Encasement 16 (like encasement 116 of FIG. 2)preferably may be made entirely of ammonia borane in order to enhancegravimetric efficiency (weight of hydrogen produced per unit weight ofencasement 16 or 116). Encasement 16 (and 116) may include othermaterials provided that such other materials are not present in suchamounts as to prevent forming the encasements into coherent,self-sustaining bodies. If such other materials are included inencasements 16 or 116, they are preferably other pyrolytic hydrides sothat they too generate hydrogen upon being heated in order to contributepositively to gravimetric efficiency. Ignition train 14 is comprised(FIG. 1A) of an ignition charge 18 in contact with pyrotechnic charge12, a pick-up charge 20 in contact with ignition charge 18, asemiconductor bridge igniter 22 mounted on a standard TO-46 header 24and embedded within pick-up charge 20. These components are allcontained within a charge holder 26. A pair of electrical leads 28 a, 28b are connected to header 24 to supply electrical power to semiconductorbridge igniter 22 from a source not shown.

Pyrotechnic charge 12 may comprise any suitable pyrotechnic material,such as a thermite. Thermite is a mixture of a reactive metal, usuallyaluminum, although other reactive metals such as manganese could beused, and an oxide such as iron oxide (Fe₃O₄ or Fe₂O₃). Aluminum hassignificant advantages as the reactive metal, including cost and ease ofhandling. The reactive metal, e.g., aluminum, and the metal oxide, e.g.,Fe₂O₃, are usually in powder form and are often mixed with a binder toprevent separation of the powders. When heated to reaction temperaturethe aluminum metal is oxidized in an aluminothermic reaction whichproduces aluminum oxide, heat at high temperatures (of up to about2,500° C. for aluminum/iron III oxides) and the metal, e.g., iron, ofthe metal oxide. Generally, such thermite materials when ignited releaseheat at temperatures on the order of about 2,000° C. to 2,500° C., havea very high caloric output to weight ratio and produce little or no gaswhen burned. Other pyrotechnics may be utilized such as, for example,various combinations of fuel and oxidizers. Exemplary fuel-oxidizercombinations are aluminum/cupric oxide; aluminum/ferric oxide;silicon/cupric oxide and silicon-boron/ferric oxide. Other combinationsof pyrotechnics can be utilized employing one or more fuels, forexample, aluminum, boron, silicon, titanium, zirconium and molybdenum incombination with one or more oxidizers such as cupric oxide, ferricoxide (Fe₂O₃), tin dioxide and titanium dioxide.

Ammonia borane is a white powder which, when placed under sufficientpressure, e.g., when subjected to a pressure of about 2,000 pounds persquare inch (“psi”) or more, will consolidate and retain its shape underits own weight. Unless specifically otherwise stated, all pressuresgiven herein or in the claims are given in pounds per square inchabsolute (“psi”). Ammonia borane pellets so produced have been handledand dropped without deformation. In addition, ammonia borane can beadded, in various weight percentages, to a pyrotechnic material toprovide different physical characteristics. With a smaller weightpercentage of ammonia borane, the mix with a pyrotechnic material, e.g.,a thermite, tends to be crusty and brittle. As the weight percentage ofammonia borane increases, the formed material becomes more ductile. Whenthe pyrotechnic charge 12 contains sufficient ammonia borane as a binderit can be formed into a coherent, self-sustaining body. The amount ofammonia borane binder thus determines the mechanical properties ofpyrotechnic charge 12. Higher levels of binder impart greater elasticityover a wide temperature range. Lower levels of binder can be useful toassist in granulation or pelletization. Properties such as crushstrength, granule size, stress, strain, and modulus are stronglyinfluenced by the amount of ammonia borane admixed with the pyrotechnicmaterial of pyrotechnic charge 12. Ammonia borane works well as binderand when used as part of a pyrotechnic charge 12 will produce hydrogenwhen the pyrotechnic charge 12 burns, thereby improving the gravimetricefficiency of the fuel element. The proportion of ammonia borane binderused in the pyrolytic charge should not, however, be so great as torender ignition, burning and attainment of desired heat output andtemperature of the pyrotechnic charge problematic. The upper limit ofammonia borane used as binder will vary depending on the ignition andburning characteristics of the particular type of pyrotechnic materialused in the pyrotechnic charge; the lower limit will depend on thedesired properties as noted above, (crush strength, granule size, etc.)of the pyrotechnic charge.

The pyrotechnic charge 12 must retain its coherence and position withinthe fuel element during manufacturing, storage, and handling. This maybe accomplished by enclosing an incoherent (e.g., powder or granular)pyrotechnic mixture within a suitable encasement, such as encasement 16described below. In addition, the pyrotechnic charge 12 may itself berendered into a coherent body, or at least a less incoherent body, byadmixing with it a sufficient quantity of a binder, preferably ahydrogen-producing binder, for example, ammonia borane, which willincrease coherence of the pyrotechnic charge. Suitable processing may beemployed to treat the admixture of binder and pyrotechnic charge, forexample, subjecting the ammonia borane to a pressure of at least about2,000 psi will form it into a coherent body. If ammonia borane is usedin sufficient quantity as the binder in the admixture of binder andpyrotechnic charge, subjecting the admixture to a pressure of, forexample, 2,000 to 10,000 psi, will render the admixture as a coherentbody. If the amount of ammonia borane binder in the admixture isinsufficient to render the admixture as a coherent, self-sustaining bodyit will at least be a less incoherent mass, i.e., it will tend toagglomerate rather than flow or separate into particles or granules.

A cavity (not shown) that retains its shape may be formed in thepyrotechnic charge 12 to allow ignition train 14 to be at least partlyembedded within pyrotechnic charge 12. Alternatively, ignition charge 18may simply be placed in abutting contact with the surface of pyrotechniccharge 12. Ignition train 14 is required to start combustion ofpyrotechnic charge 12. Adding a binder to pyrotechnic charge 12 is onemeans of ensuring that the pyrotechnic charge 12 is a coherent bodywhich can be retained within the fuel element and against or withinwhich an ignition train 14 can be mounted. The use of mechanical meanssuch as a metal or plastic (synthetic organic polymer) housing orcontainer or the like for such purpose is not practical for devicesrequiring high gravimetric efficiency (mass of hydrogen producedrelative to overall mass of the device). One of the biggest drawbacks ofusing inert or reactive but non-hydrogen producing binders or mechanicalhousings in this device, is that such binders or housing would reducethe gravimetric efficiency of the system by adding to the system masswhich does not produce hydrogen. Another drawback is that suchexpedients reduce hydrogen generating efficiency by absorbing heatneeded to pyrolyze the hydrogen solid fuel, e.g., ammonia borane. Manynon-reactive binders can also produce gaseous species (chlorine- andfluorine-based compounds) that are harmful, e.g., to a fuel cell systemto which the evolved hydrogen is supplied. Reactive binders used withpyrotechnic materials produce gaseous products (carbon monoxide andnitric oxides) that are also harmful to fuel cells. Although a fewenergetic binders exist, the majority of the chemical binders absorbheat from the pyrotechnic reaction. All such chemical binders add masswhich does not produce hydrogen.

Encasement 16 preferably is comprised of substantially pure ammoniaborane and may be formed into a coherent, self-sustaining cup-like bodyof sufficient structural strength to contain pyrotechnic charge 12 withignition train 14 mounted thereon. The cold flow properties of ammoniaborane permit forming it into a coherent, self-sustaining encasementwithout need for additives of any kind, thereby providing an encasementof high gravimetric efficiency. Although not preferred, in otherembodiments, encasement 16 may comprise ammonia borane admixed withother materials, preferably with other hydrogen-evolving materials suchas other pyrolytic hydrides, e.g., hydrazine bis-borane, N₂H₄(BH₃)₂.Encasement 16 contains at least a sufficient percentage by weight ofammonia borane to insure that encasement 16 is capable of being formedinto a coherent, self-sustaining body of sufficient structural strengthand coherence to serve as an encasement for pyrotechnic charge 12.

The ammonia borane encasement 16 produces hydrogen when the hydrogenfuel element 10 is functioned. Encasing the pyrotechnic charge 12 inammonia borane optimizes the use of heat from the pyrotechnic charge inthat pyrotechnic-generated heat must flow through ammonia borane.

Utilizing ammonia borane as the binder for pyrotechnic charge 12 and asthe sole or major component of encasement 16 will contribute to theoverall production of hydrogen while generating few destructive gaseousspecies. Combined with a suitable pyrotechnic, e.g., a thermite inpyrotechnic charge 12, the ammonia borane binder material is heated totemperatures high enough to remove hydrogen from the ammonia borane. Theintimate admixture of ammonia borane binder and the pyrotechnic materialparticles of pyrotechnic charge 12 insures good heat transfer to theammonia borane upon ignition of the pyrotechnic charge, whichfacilitates heating the ammonia borane to a temperature high enough toattain removal of all three mols of hydrogen from the ammonia borane. Asthose skilled in the art will appreciate, removal of the third mol ofhydrogen requires attainment of a higher temperature than that requiredto remove the first two mols of hydrogen, and the presence of anappropriate proportion of ammonia borane binder in intimate admixturewith a suitable pyrotechnic, e.g., thermite, facilitates the attainmentof such high temperature by the ammonia borane. The ammonia borane usedto form the pellet-like pyrotechnic charge 12 and encasement 16 is alsoa, or the, primary reactant for the hydrogen generation. The unique“cold flow” properties of ammonia borane allow not only for forming acoherent, self-sustaining encasement, but also allows for pelletizationof a mixture of ammonia borane particles with particles of a suitablepyrotechnic material, in cases where solid hydrogen fuel elements havingthe form of pellets having good mechanical properties are desired.

In operation of the solid hydrogen fuel element 10 of FIG. 1, thesemiconductor bridge igniter 22 is supplied with electrical powerthrough leads 28 a, 28 b and generates a plasma which ignites pick-upcharge 20. The pick-up charge 20 in turn ignites the ignition charge 18which ignites the pyrotechnic charge 12. As the pyrotechnic charge 12burns, its energy is transferred to the ammonia borane (a pyrolytichydride) contained in pyrotechnic charge 12 and in encasement 16,causing the ammonia borane to decompose and give off hydrogen.

Referring now to FIG. 2, there is shown an embodiment of the inventionin which a hydrogen fuel element 110 comprises an encasement 116 ofammonia borane which entirely encloses the pyrotechnic charge 12.(Components of the several embodiments illustrated in the Figures whichare identical are identically numbered; corresponding components whichare not identical are numbered by adding 100 to the counterpart elementof the other embodiments.) The only opening in encasement 116 is closedby an ignition train 14; the passage of ignition train 14 (or any otherdevice) through encasement 116 and into contact with pyrotechnic charge12 is not deemed to change the fact that encasement 116 “fully encases”pyrotechnic charge 12. The quoted term is deemed to embrace structuresin which the encasement is penetrated by an ignition train or the like.Ignition train 14, which may be identical to that of FIG. 1A, is, in theillustrated embodiment, mounted in contact with pyrotechnic charge 12 toinsure that ignition train 14 is in signal transfer communication withpyrotechnic charge 12. The term “signal transfer communication” in thiscontext means merely that functioning of ignition train 14 will ignitepyrotechnic charge 12. Ignition train 14 has an output end (provided inthe illustrated embodiment of FIG. 1A by ignition charge 18) whichcontacts and which optionally may penetrate into pyrotechnic charge 12so as to be partly embedded therein, to promote good signal transfer to,and ignition of, pyrotechnic charge 12. Electrical leads 28 a, 28 bprotrude from encasement 116. This fully-encased arrangement improvesfunctional reliability while improving the gravimetric hydrogenproduction efficiency by including additional ammonia borane.

The encasement 116 of ammonia borane will also keep the ignition train14 in compression, thus keeping all energetic interfaces (betweensemiconductor bridge igniter 22, pick-up charge 20, ignition charge 18and pyrotechnic charge 12) in intimate contact with each other. Evensmall gaps (0.005″ or less) between these elements can cause disruptionsin the propagation and cross propagation of the reaction betweenadjacent ones of the different energetic materials.

Electrical leads 28 a, 28 b protruding from the encasement 116 allow anelectrical signal to be sent to the ignition train 14 to start theproduction of hydrogen. The ammonia borane encasement 116 (likeencasement 16 of FIG. 1) provides structural support duringmanufacturing, storage and operation and provides containment whichkeeps the pyrotechnic charge 12 in intimate contact with the ignitioncharge 18 of ignition train 14.

Referring now to FIG. 3, there is shown an embodiment of the presentinvention in which the solid hydrogen fuel element 210 further comprisesa housing 30 within which is contained a fuel element similar oridentical to element 110 of FIG. 2. Housing 30 may be made of anysuitable material having sufficient structural strength and permittingthe passage therethrough, via openings (not shown) or via a porous orforaminous structure, of hydrogen gas generated by functioning ofhydrogen fuel element 210. For example, housing 30 may comprise a hightemperature refractory carbon foam comprised of open-cell pores. Suchmaterial not only will allow generated hydrogen to flow through theporous structure of housing 30, but will capture particulate and liquidphase reaction products, and retain heat from the pyrotechnic charge 12while slowly distributing heat to control the temperature within thehousing 30. The foam has a low thermal conductivity, a pore size capableof retaining solids and liquid (formed during the functioning of thedevice), allows hydrogen to freely flow through it, and has a lowdensity. The foam material is machinable but can be molded to shapeduring manufacturing and/or machined to final dimensions andconfiguration. Carbon foams have a lower density relative to other solidporous materials and so the weight penalty imposed by this non-hydrogengenerating material is limited. The foam manufacturing process can betailored to change the pore size and the thermal insulating propertiesof the foam. Fibers can be incorporated into the carbon foam and thesefibers act as condensing surfaces for liquid reaction products. Many ofthe condensed reaction products can still give off hydrogen ifsufficient heat is evolved by pyrotechnic charge 12 to maintain asufficiently high temperature.

Generally, it is seen that the hydrogen fuel element comprises apyrotechnic charge, an ignition train in energy-transfer relationshipwith the pyrotechnic charge, which is wholly or partly encased within anammonia borane encasement and may include an ammonia borane binder. Thehydrogen fuel element may also include a suitable outer casing orhousing, such as a carbon foam housing.

The fuel elements 10, 110 and 210 may be manufactured by molding ammoniaborane powder at a pressure of from about 2,000 to 10,000 psi to form acoherent, self-sustaining body generally in the form of an open cup asillustrated in FIG. 1. A pyrotechnic charge 12 may then be placed withinthe formed cup of ammonia borane. The ammonia borane may be left withinthe mold if it is desired to press the pyrotechnic material to compactit. The pyrotechnic charge 12 may contain a pyrolytic hydride binder,such as ammonia borane. For a full ammonia borane encasement asillustrated in FIG. 2, the open ammonia borane cup is closed by pressinga layer of ammonia borane over the open end, thereby enclosingpyrotechnic charge 12 within the full encasement. In this embodiment,the ignition train 14 may be disposed within the closing layer ofammonia borane. In all cases, the ignition train is in signal transfercommunication with, preferably in abutting contact with or slightlyembedded within, pyrotechnic charge 12 to insure good contact.

For production of the hydrogen fuel element as illustrated in FIG. 3, asuitable housing, which may be a high-temperature refractory carbon foamas described above, may be formed by any suitable means to encloseammonia borane encasement 116.

Housing 30 provides a more robust structure as well as enhancing areliable contact between elements of the ignition train 14. The housing30, however, unavoidably reduces the gravimetric efficiency of thehydrogen fuel element 210. The ammonia borane encasement 116 andpyrotechnic charge 12 can be pressed into or poured into the housing 30.When the materials are ammonia borane-containing powders which arepressed into housing 30, the powder will bind within the housing 30,creating a residual stress at the interface of the walls of housing 30and the pressed powder. The residual stress keeps the material intactwithin the housing 30, thus preventing the materials from moving duringstorage, transportation, or operation. For pressed powders, the housingprovides structural support during the pressing process.

FIGS. 4A-4E illustrate steps in the manufacture of solid hydrogen fuelelement 10 of FIG. 1. A mold 32 has a charge 34 of ammonia borane placedtherein as shown in FIG. 4A. FIG. 4B shows a die 36 inserted within mold32 to apply sufficient pressure, e.g., a pressure of about 2,000 toabout 10,000 psi, to form the incoherent charge 34 into a coherent,self-sustaining encasement 16 within mold 32, as shown in FIG. 4C. Acharge 38 of pyrotechnic material is introduced into encasement 16 whilethe latter is still retained within mold 32, as shown in FIG. 4D. Asuitable binder, preferably a pyrolytic hydride binder, most preferablyammonia borane, is included in charge 38 so that upon application ofpressure by die 36 to charge 38 to form pyrotechnic charge 12 asillustrated in FIG. 4E, the coherence of pyrotechnic charge 12 isincreased, preferably to the point of rendering it as a coherent body.In some cases, the pyrotechnic charge may be left in a partlyagglomerated state, i.e., may not be formed into a coherent,self-sustaining body. In other cases, pyrotechnic charge 12 may be leftin a granular or powder form.

If it is desired to manufacture the solid hydrogen fuel element 110 ofFIG. 2, another processing step (not illustrated) is utilized to encasethe exposed surface of pyrotechnic charge 12 (FIG. 4E) with a coherentlayer of ammonia borane. Upon the application of suitable pressure, aclosing layer of ammonia borane can be sealed to the lip of the “cup” ofammonia borane encasement 16 while the latter is in the mold 32. Asuitable opening may be left in, or made in, the closing layer ofammonia borane to receive an ignition train such as ignition train 14 ofFIG. 1A. Alternatively, some other suitable cover may be applied. It ispreferred, however, to provide an encasement made entirely of ammoniaborane as illustrated in FIG. 2 for enhanced gravimetric efficiency.Subsequent manufacturing steps (not illustrated) are utilized to add tohydrogen fuel elements 10, 110 or 210 an ignition train 14 and, in thecase of hydrogen fuel element 210, a housing 30.

Generally, any suitable pyrotechnic material can be utilized to provideheat to the pyrolytic hydride. The pyrotechnic material should have ahigh energy density, be ignitable by a semiconductor bridge device (orother energetic charge), yield insignificant gas production, beenvironmentally green (before and after combustion), burn in an inertatmosphere at atmospheric pressure, and retain its shape duringmanufacturing, storage and operation. A number of energetic materials(thermites, heat powders and intermetallic mixes) exist which have ahigh energy density, low gas output, manageable ignition properties, arecapable of sustaining combustion in an inert atmosphere at atmosphericpressure, and are environmentally acceptable. These materials requirebinders or high pressure to create the configurations shown in thefigures.

The ammonia borane will produce hydrogen as the pyrotechnic chargeburns. Due to the very high temperature (greater than 2,000° C.)generated during the combustion of the pyrotechnic charges, and theclose proximity of the ammonia borane to the pyrotechnic charge, e.g.,thermite, a high proportion of the hydrogen contained in the ammoniaborane is released.

While the invention has been described in detail with respect to aspecific embodiment thereof, it will be appreciated that the inventionhas other applications and may be embodied in numerous variations of theillustrated embodiment.

1. A solid hydrogen fuel element comprising a pyrotechnic charge and acoherent, self-sustaining ammonia borane encasement at least partlyencasing the pyrotechnic charge.
 2. The fuel element of claim 1 whereinthe encasement fully encases the pyrotechnic charge.
 3. The fuel elementof claim 1 or claim 2 wherein the pyrotechnic charge includes a binderwhich upon being heated to its activation temperature releases hydrogen.4. The fuel element of claim 3 wherein the binder comprises ammoniaborane.
 5. The fuel element of claim 1 or claim 2 wherein the ammoniaborane encasement is made by a process of molding ammonia borane into ahollow shape by placing the ammonia borane into a suitably shaped moldand applying sufficient pressure to the ammonia borane within the moldto render the encasement as a coherent, self-sustaining body, andplacing the pyrotechnic charge within the encasement.
 6. The fuelelement of claim 5 including applying a pressure of at least about 2,000psi.
 7. The fuel element of claim 5 including applying a pressure offrom about 2,000 to 10,000 psi.
 8. The fuel element of claim 5 whereinthe pyrotechnic charge is made by a process of admixing an incoherentpyrotechnic charge with a binder to form an admixture of the binder andthe incoherent pyrotechnic charge.
 9. The fuel element of claim 6wherein the binder is present in the admixture in a quantity which issufficient, upon application of sufficient pressure to the admixture, torender the incoherent pyrotechnic charge as a coherent, self-sustainingbody, but which quantity is not so great as to preclude reliableignition and burning of the pyrotechnic charge.
 10. The fuel element ofclaim 9 including applying to the admixture a pressure of at least about2,000 psi.
 11. The fuel element of claim 10 wherein the binder comprisesammonia borane.
 12. The fuel element of claim 8 wherein the pyrotechniccomposition comprises one or more fuel/oxidizer couples and wherein thefuel is selected from the group consisting of one or more of aluminum,boron, silicon, titanium, zirconium and molybdenum, and the oxidizer isselected from one or more of cupric oxide; Fe₃O₄; Fe₂O₃; tin dioxide andtitanium dioxide.
 13. The fuel element of claim 8 wherein thepyrotechnic composition is selected from the group consisting of one ormore of the following fuel/oxidizer couples: aluminum/cupric oxide;aluminum/Fe₃O₄; aluminum/Fe₂O₃; silicon/cupric oxide;silicon-boron/Fe₃O₄ and silicon-boron/Fe₂O₃.
 14. The fuel element ofclaim 8 wherein the pyrotechnic material comprises a thermite.
 15. Thesolid hydrogen fuel element of claim 1 or claim 2 further comprising anignition train positioned in energy transfer relationship with thepyrotechnic charge.
 16. The fuel element of claim 15 wherein theignition train is embedded in the encasement.
 17. The fuel element ofclaim 15 wherein the ignition train is positioned within the pyrotechniccharge.
 18. The fuel element of claim 15 wherein the ignition train isembedded within the encasement and extends into contact with thepyrotechnic charge.
 19. The fuel element of claim 15 further comprisinga housing enclosing the ammonia borane encasement, which housing ispervious to hydrogen gas generated by the fuel element.
 20. A method ofmaking a solid hydrogen fuel element comprising subjecting ammoniaborane to a pressure sufficient to form it into a coherent,self-sustaining hollow encasement of ammonia borane, and at least partlyencasing a pyrotechnic material within the encasement.
 21. The method ofclaim 20 including fully encasing the pyrotechnic material within theammonia borane encasement.
 22. The method of claim 20 or claim 21wherein the pressure is at least about 2,000 psi.
 23. The method ofclaim 20 or claim 21 wherein the pressure is from about 2,000 to about10,000 psi.
 24. The method of claim 20 or claim 21 further comprisingadmixing a binder with the pyrotechnic material, the binder comprising apyrolytic hydride characterized by evolving hydrogen at least whenheated to a temperature sufficiently high to evolve hydrogen fromammonia borane.
 25. The method of claim 20 or claim 21 furthercomprising mounting an ignition train in signal transfer communicationwith the pyrotechnic charge.
 26. The method of claim 25 wherein theignition train is mounted within the ammonia borane encasement.
 27. Themethod of claim 25 wherein the ignition train has an output end andmounting the output end in contact with the pyrolytic material.
 28. Themethod of claim 25 further comprising enclosing the ammonia boraneencasement within a housing which is pervious to the flow of hydrogengas from the interior to externally of the housing.